The present invention relates to a radiographic diagnosis apparatus which irradiates an object with radiation such as X-rays from a radiation source and generates a radiographic diagnosis image by detecting the radiation transmitted through the object and, more particularly, to a radiographic diagnosis apparatus which uses a two-dimensional array detector, in which a plurality of detecting elements are arrayed in a two-dimensional matrix, as a radiation detector, irradiates an object with a plurality of radiations differing in energy, and reconstructs a desired image by using the energy differences.
This application is based on Japanese Patent Application No. 10-326993, filed Nov. 17, 1998, the entire content of which is incorporated herein by reference.
One conventional radiographic diagnosis apparatus using radiation such as X-rays is a technique which acquires images by using two different X-ray energies (high and low energies) and performs energy subtraction by linear arithmetic operations (weighted differential processing) for the images (William R. Brody et. al., "A method for selective tissue and bone visualization using dual energy scanned projection radiography", Med. Phys. 8(3), May/June 1981). The purpose of this technique is to display only a soft tissue by erasing information, such as a bone, unnecessary for diagnosis by subtraction and to thereby allow easy diagnosis of a soft tissue hidden behind a bone.
Also, a technique by which images are acquired by a plurality of different X-ray energies, not by two different energies, and the energy absorption characteristic of a substance is visualized by linear arithmetic operations for the images is described in Japanese Patent Application No. 3-334788 assigned to the same assignee as the application concerned. The purpose of this technique is to visualize the differences between X-ray energy spectrum absorbed by, e.g., bones, soft tissues, and lungs. Since the method of display is different from the one that displays X-ray attenuation amounts, the method is expected to be applied to tissue characterization.
As a means for acquiring a plurality of different energy images, a method of acquiring images by emitting a plurality of different radiations at different timings is known. This method sequentially emits a plurality of different radiations and acquires a plurality of image data by detecting the emitted radiations by a single detector. In this method, the same object is irradiated at least twice at different timings with different radiations. Therefore, if the object moves, the data acquisition positions of the second and subsequent radiations deviate from that of the first radiation. This generates an artifact and thereby degrades the image quality.
To prevent this, methods are being developed by which a plurality of different radiations are simultaneously emitted and detected by a plurality of different detectors. One example is a method which uses a detecting device formed by overlapping a plurality of detectors with a substance which changes X-ray properties sandwiched between them. This method emits one type of radiation, changes the radiation properties by passing it through the substance, and acquires a plurality of images at the same time by the first detector and the second and subsequent detectors. Since a plurality of different radiations are simultaneously emitted and a plurality of images are simultaneously acquired, no such artifact as caused by the motion of an object as described above is produced. However, a detector (first detector) placed nearest to an X-ray generator must transmit X-rays to a certain degree to a subsequent detector (second detector), i.e., must not absorb X-rays 100%. Also, the second detector detects X-rays attenuated by the first detector, so the incident dose is reduced compared to that to the first detector. As a consequence, the dose detected by each detector reduces, and this decreases the ratio of the effective dose to noise, i.e., the S/N ratio of an image; the influence of noise increases to degrade the image quality. Furthermore, since the detectors overlap each other in the form of a sandwich, scattering rays generated by the individual detectors have influence on each other. This degrades the image quality acquired by each detector.
An array detector in which small detecting elements are arrayed in a two-dimensional matrix has been developed recently as an X-ray detector (e.g., U.S. Pat. No. 4,672,454). This detector is characterized in that the positions of pixels corresponding to the pixels of a digital image are spatially determined. Feasibility of applying the aforementioned diagnosis methods such as energy subtraction to a detector having this two-dimensional array structure is under consideration.
This X-ray array detector, however, has a structure in which detecting elements are arrayed on the detector. Hence, it is necessary to lay in electric wires, for propagating input control signals to the detecting elements and output electrical signals from the detecting elements, inside the detector. If this detector is constructed into the shape of a sandwich, therefore, the shadow of the wiring in the first detecting element is projected onto the second detecting element to generate an artifact. This makes this detector difficult to put into practice.
As described above, no conventional means exists which acquires a plurality of different energy images without generating any artifact, reducing the S/N ratio, and degrading the image quality by scattering rays.